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Ti:Sapphire Lasers
Tyler Bowman
April 23, 2015
Introduction
Ti:Sapphire lasers are a solid state laser group based on using titanium-doped sapphire (Ti:Al2O3)
plates as a gain medium. These lasers are very popular for a wide range of applications both
commercially and for research due to their broad tunability, high stability, and significant potential
output power. [1],[2] Additional research into Ti:sapphire has shown it to have a very short upper carrier
lifetime, making it ideal for very high quality time-domain pulses. [3] Ti:Sapphire lasers have been shown
to overtake many applications of dye lasers due to having similarly large ranges of frequency tuning in a
solid-state device instead of a liquid dye. As such, there have been a wide number of applications that
have begun to use Ti:sapphire and other crystal-based gain materials for lasers.
The first successfully observed lasing of Ti:sapphire was reported by Moulton in 1986, where it
was found that pumping with dye lasers, Nd:YAG lasers, and argon-ion lasers provided a tunable range
from 660 nm to 986 nm. [4] The first commercial model for mode-locked time-domain pulsing was
released in 1990, while the first continuous wave system was developed in 1998. [3] From that time
Ti:sapphire lasers have expanded into a wide number of applications and research areas.
Laser Properties
The table in [3] gives a good number of properties of the gain medium for Ti:Sapphire systems.
Property
chemical formula
crystal structure
melting point
thermal conductivity
thermal expansion coefficient
thermal shock resistance parameter
refractive index at 633 nm
temperature dependence of refractive index
Ti density for 0.1% at. doping
fluorescence lifetime
emission cross section at 790 nm
Value
Ti3+:Al2O3
hexagonal
2040 °C
33 W / (m K)
≈ 5 × 10−6 K−1
790 W/m
1.76
13 × 10−6 K−1
4.56 × 1019 cm−3
3.2 μs
41 × 10−20 cm2
Table 1: Standard properties of Ti:Sapphire [3]
It has been additionally reported that Ti:Sapphire has a good quantum efficiency at room
temperature around 0.7 [5], with some estimations going as high as 0.8. [1] It is generally accepted in
literature that pulsed Ti:Sapphire systems produce an output power of 0.1 to 3 W of power, while
continuous wave systems can maintain a power of several watts. The pulse width of Ti:Sapphire systems
is generally on the order of 100 fs, though several research setups of the lasers have approached widths
of 5 fs or less. [3] Additionally, a wide range of values for continuous wave Ti:Sapphire lasers were
performed in [1], where the laser was approximated as a 4-level system that was then reduced to a 2level approximation. Within this system the upper state decay rate 𝛾2 was stated to be 288 kHz (with a
corresponding decay time of 𝜏2 = 3.4 µs. This is consistent with the properties reported in Table 1.
Additional information found by the experimental analysis of the circular continuous wave cavity are the
cavity loss and gain per pass, with the gain defined in terms of the incoming pump power these values
are 3.6 % loss per pass in the cavity and 0.707% gain per watt per pass. Thus for that particular setup a
pump power of around 5 W was required in order to achieve threshold inversion, though this value
would change depending on the intracavity losses.
Ti:Sapphire Fabrication
The standard method for creating the Ti:Sapphire crystal used in solid state lasers is standard
semiconductor processing. For doping sapphire with titanium ions, this generally involves melting down
Al2O3 (sapphire) crystals in a crucible and infusing the melt with TiO2 (titanium oxide). The selection of
this particular oxide is in order to get the necessary Ti3+ ions when the oxide bond is broken. A seed
crystal of Al2O3 is then dipped into the mix and slowly drawn out such that the uniform sapphire crystal
solidifies onto the seed. Once a rod of doped Ti:Al2O3 has been pulled from the melt, it can then be
sliced and sectioned into whatever size is needed This particular method is known as the Czochralski
growth technique and is commonly used in the semiconductor industry. However, there are many other
techniques that are effective in obtaining these crystals as well. [6]
Laser Cavity Construction
The construction of the Ti:sapphire laser cavity has a very similar structure to most dye laser
systems or other regions using a solid gain medium. A sample figure of the laser cavity is given in Fig. 1.
Fig. 1: Basic Ti:Sapphire Laser setup (Image sourced from
http://www.bradallica.com/HET417/Titanium_Sapphire_Lasers.htm)
This image gives an example of a ring cavity Ti:sapphire laser excited by an arbitrary pump
beam. This setup can also be realized as a resonant cavity in which the signal passes through the
Ti:sapphire crystal gain medium twice per round trip through the cavity. However, the presence of
intersecting waves in the gain medium can cause spatial hole burning that exhausts the gain medium.
Additionally, a resonant cavity has the risk of sending interfering signals back to the pump laser. Finally
there is some need for additional space for other components. On the other hand, aligning the beam
waist within the Ti:Sapphire crystal for optimized gain is more feasible in the resonant cavity setup. Thus
there are some benefits and drawbacks to either setup such that the type of cavity must be chosen
based on the specific needs of the laser being built.
Within the sample cavity of Fig. 1 there are several additional components in the optical path
other than the gain medium and the mirrors. Each of these is a standard component for solid state or
dye lasers in order to address some basic considerations for the design. The first object to be addressed
is the pump lens, which is needed for mode matching inside of the cavity. Since the exact waist position
can be difficult to arrange in the center of the gain medium using the cavity mirrors alone, this focusing
lens serves the purpose of bringing the pump signal to a focus in the middle of the crystal. This
guarantees a strong and stable gain. [1]
The second aspect being addressed is the optical diode, which serves the purpose of limiting the
lasing to a single direction within the cavity. While it can be loosely assumed that any fluorescence from
the crystal will be parallel with the incident pump signal, this is not the case in practice and there will be
some fluorescence traveling the opposite direct through the cavity from the primary lasing signal. In
order to eliminate this backwards-traveling signal, a birefringence layer or Faraday rotator is used to
change the polarization of the waves that travel through it. The signals are then passed through a half
wave plate. In this way, waves traveling in one direction can be given a rotation that eliminates the
signal when it passes into the half wave plate, whereas the waves traveling in the other direction will
simply change polarization and continue. [1]
The third component to note is the standalone birefringent tuner in the optical path. This tuner
consists of a material that is nonlinear at optical frequencies, and it is arranged in such a way that it is at
a Brewster angle for the wavelength of emission being tuned. Given the nature of the nonlinear
medium, any frequencies outside of the tuned frequency are either absorbed or redirected outside of
the optical path of the system. In this way the desired frequency of the Ti:sapphire laser is isolated while
the other frequencies of the laser’s gain bandwidth are tuned out. The final additional component of the
ring system is the etalon, which isolates the lowest order mode to propagate while absorbing the rest.
[1]
The functions of the additional components within the system are generally consistent even if
the specific devices used vary. Specifically, components for the isolation of a single wavelength, the
enforcement of a single direction of lasing, and the reduction to a single mode are all key aspects of an
effective Ti:Sapphire laser cavity.
In addition to standard continuous wave activity in the cavity, one of the more attractive aspects
of the Ti:Sapphire laser is a very robust ability to be in mode-lock. In short, mode-locking makes use of
nonlinear optical systems in order to align the phase components of the signal. This alignment of the
signal across a number of frequencies creates a constructive time domain pulse signal. Due to the large
gain bandwidth of the Ti:Sapphire emission, these lasers are capable of a very refined pulse with a
FWHM as small as 5.5 fs [3],[7]. This mode-lock can be achieved in several ways, but all methods take
the role of an effective saturable absorber. [8] These setups generally involve actual saturable
absorbers, Kerr lensing, and gain modulator manipulation of the signals, but the final goal is to obtain a
group of frequencies within the gain bandwidth that are in phase. The use of Kerr lensing in particular
has shown effectiveness in increasing the output power and decreasing the pulse width over saturable
absorber reflectors. [9] The use of a mode-locked, pulsing signal opens up a wide number of applications
that are not possible with a continuous wave laser.
Applications
One area of interest uses the wide tunable range of Ti:sapphire lasers in order to pump other
laser and optical sources. This technology has been applied for exciting optical-band elements as well as
other frequency ranges of interest like terahertz and X-ray signal generation. In particular, the very
narrow Ti:Sapphire peak is useful for exciting semiconductor devices and antennas to produce terahertzfrequency pulses representing wide frequency ranges. For increasing the frequency to UV and X-ray
frequencies, frequency multiplication of the Ti:Sapphire pulses allow the generation of signals not
possible by any other laser system. [8] Other applications include the observance of chemical transitions
and other optical properties of materials in very rapid time scales. Additionally, Ti:sapphire lasers have
shown particular suitability to multiphonon microscopy due to a high peak power and tight focus. In this
application, a narrow peak and small focal point allow for highly specified fluorescence of chemical dyes.
The precision of these measurements allows for higher resolution detection while avoiding potential
bleaching of the dyes from overexposure. [10] Finally, Ti:Sapphire pulses have been shown to be useful
in detecting chemical transitions on the femtosecond scale by exciting individual molecules. [11] In
summary, Ti:sapphire lasers have been shown to be highly suitable to many applications.
Conclusion
Since their first development, Ti:sapphire lasers have shown a strong potential for a wide array
of applications due to having a broad gain bandwidth resulting in a highly tunable continuous wave
system or a narrow pulsed system in mode-lock. Due to its high flexibility, Ti:sapphire has become one
of the most commonly used laser systems in both continuous wave and pulsed systems.
References
[1]
W. L. Erickson and S. P. Singh. “System Design and Relaxation Oscillations of a Titanium-Sapphire
Laser.” MS Thesis. University of Arkansas. 1992.
[2]
J. Klein. “The Ti:Sapphire Laser: From Research to Industry and Beyond.” Photonics Spectra.
Published online. Accessed on 2015-04-23. <http://www.photonics.com/Article.aspx?AID=42950>
[3]
R. Paschotta. “Titanium-sapphire Lasers.” Encyclopedia of Laser Physics and Technology,
<http://www.rp-photonics.com/titanium_sapphire_lasers.html>, accessed on 2015-04-23.
[4]
P.F. Moulton. “Spectroscopic and laser characteristics of Ti:Al2O3.” J. Opt. Soc. Am. B, vol. 3, no. 1,
January 1986.
[5]
P. Albers, E. Stark, and G. Huber. “Continuous-wave laser operation and quantum efficiency of
titanium-doped sapphire.” J. Opt. Soc. Am. B, vol. 3, no. 1, January 1986.
[6]
R. Uecker, D. Klimm, S. Ganschow, P. Reiche, R. Bertram, M. Roßberg. “Czochralski growth of
Ti:sapphire laser crystals.” Proceedings of SPIE 10. 2005.
[7]
K.F. Wall and A. Sanchez. “Titanium Sapphire Lasers.” The Lincoln Laboratory Journal, vol. 3, no. 3,
1990.
[8]
G. Steinmeyer. “A review of ultrafast optics and optoelectronics.” J. Opt. A: Pure Appl. Opt., vol. 5,
pp. R1-R15, 2003.
[9]
C.G. Durfee, T. Storz, J. Garlick. S. Hill, J.A. Squier, M. Kirchner, G. Taft, K. Shea, H. Kapteyn, M.
Murnane, and S. Backus. “Direct diode-pumped Kerr-lens mode-locked Ti:sapphire laser.” OPTICS
EXPRESS, vol. 20, no. 13, pp. 13677-13683, 18 June 2012.
[10] M.D. Young, S. Backus, C. Durfee, and J. Squier. “Multiphonon Imaging with a Direct-diode
Pumped Femtosecond Ti:sapphire Laser.” J. Microsc., vol. 249, no. 2, pp. 83-86, February 2013.
[11] A. Talbpour, A.D. Bandrauk, J. Yang, S.L. Chin. “Multiphoton ionization of inner-valence electrons
and fragmentation of ethylene in an intense Ti:sapphire laser pulse.” Chemical Physics Letters, vol.
313, pp. 789-794, 1999.